Mutations can
result from
one change in
one nucleotide
of 6 billion in a
human cell.

Can
sex earn its keep?

But why eliminate mutations in this way, rather than
correcting more of them by better proofreading? Kondrashov
has an ingenious explanation of why this makes sense:
It may be cheaper to allow some mistakes through
and remove them later. The cost of perfecting proofreading
mechanisms escalates as you near perfection.

According to Kondrashov's calculations, the rate of
deleterious mutations must exceed one per individual
per generation if sex is to earn its keep eliminating
them; if less than one, then his idea is in trouble.
The evidence so far is that the deleterious mutation
rate teeters on the edge: it is about one per individual
per generation in most creatures. But even if the rate
is high enough, all that proves is that sex can perhaps
play a role in purging mutations. It does not explain
why sex persists.

The
main defect in Kondrashov's hypothesis is that it works
too slowly. Pitted against a clone of asexual individuals,
a sexual population must inevitably be driven extinct
by the clone's greater productivity, unless the clone's
genetic drawbacks can appear in time. Currently, a great
deal of effort is going into the testing of this model
by measuring the deleterious mutation rate, in a range
of organisms from yeast to mouse. But the answer is still
not entirely clear.